Centrifugal and axial flow compressors include a fluid inlet, a fluid outlet and one or more arrays of compressor blades projecting outwardly from a rotatable hub or shaft. A casing, whose inner surface defines the outer boundary of a fluid flowpath, circumscribes the blade arrays. Each compressor blade spans the flowpath so that the blade tips are proximate to the outer flowpath boundary, leaving a small clearance gap to enable rotation of the shaft and blades. During operation, the compressor pressurizes a stream of working medium fluid, impelling the fluid to flow from a relatively low pressure region at the compressor inlet to a relatively high pressure region at the compressor outlet.
Because compressors urge the working medium fluid to flow against an adverse pressure gradient (i.e. in a direction of increasing pressure) they are susceptible to stall, a localized fluid dynamic instability that locally impedes fluid flow through the compressor and by surge, a larger scale fluid dynamic instability characterized by fluid flow reversal and disgorgement of the working medium fluid out of the compressor inlet. Compressor stall and surge are obviously undesirable. If the compressor is a component of an aircraft gas turbine engine, a surge is especially unwelcome since it causes an abrupt loss of engine thrust and can damage critical engine components.
In a turbine engine, surge or stall may be provoked by any of a number of influences, among them fluid leakage through the clearance gap separating each blade tip from the compressor case. Leakage occurs because the fluid pressure adjacent the concave, or pressure surface of each blade exceeds the pressure along the convex, or suction surface of each blade. The leaking fluid interacts with the fluid flowing through the primary flowpath to form a fluid vortex. The strength of the vortex depends in part on the size of the clearance gap and on the pressure difference or loading between the suction and pressure sides of the blade. Compressors can usually tolerate vortices of limited strength. However a locally excessive clearance gap or locally excessive loading of one or more blades can generate a vortex powerful enough to seriously disrupt the progress of fluid through the flowpath, resulting in a surge or stall.
Compressor designers strive to develop compressors that are highly tolerant of potentially destabilizing influences. One way that designers enhance compressor stability is by incorporating special features, referred to as casing treatments, in the compressor case. One type of stability enhancing casing treatment is a series of circumferentially extending grooves, each substantially perpendicular to the streamwise direction (the predominant direction of fluid flow in the flowpath). U.K. Patent Application 2,158,879 depicts such a casing treatment, but does not elaborate on the physical mechanisms responsible for improving stability. It is thought that the grooves provide a means for fluid to exit the flowpath at a locale where the blade loading is severe and the local pressure is high, migrate circumferentially to a locale where the pressure is more moderate, and re-enter the flowpath. The migrated fluid is thus better positioned to contend with the adverse pressure gradient in the flowpath. Moreover, the fluid migration helps relieve the locally severe blade loading. It has also been observed that the presence of the grooves degrades compressor efficiency, presumably because fluid re-enters the flowpath in a direction substantially perpendicular to the streamwise direction, resulting in efficiency losses as the re-entering fluid collides with and mixes turbulently with the flowpath fluid stream. The re-entering fluid, lacking any appreciable streamwise directional component of its own, may also tend to recirculate unbeneficially into and out of the groove.
Another type of casing treatment is shown in U.S. Pat. No. 5,762,470 and U.K Patent Application 2,041,149. These patents disclose compressors employing a manifold to alleviate circumferential pressure nonuniformities that may be associated with destabilizing tip leakage vortices. The manifold shown in U.S. Pat. No. 5,762,470 is an annular cavity that communicates with the flowpath by way of a series of slots separated by a gridwork of ribs. U.K. Patent Application 2,041,149 discloses a centrifugal compressor having a manifold that communicates with flowpath through a set of slotted diffuser vanes. The application also discloses an axial flow compressor with a manifold radially outboard of the compressor flowpath and a manifold chamber radially inboard of the flowpath. A spanwise slot on the suction surface of each compressor blade places the compressor flowpath in fluid communication with the inboard manifold chamber. The compressor vanes include similar slots that connect the flowpath to the outboard manifold. Notwithstanding the possible merits of the disclosed arrangements, they clearly introduce a measure of undesirable manufacturing complexity into the compressor.
Still another type of casing treatment is shown in U.S. Pat. Nos. 5,282,718, 5,308,225, 5,431,533 and 5,607,284, all of which are assigned to the assignee of the present application. These patents describe variations of a turbine engine casing treatment known as vaned passage casing treatment (VPCT). The disclosed casings include a passageway occupied by a set of anti-swirl vanes. Fluid extraction and injection passages place the vaned passageway in fluid communication with the compressor flowpath. During operation, fluid with degraded axial momentum, but high tangential momentum, flows out of the flowpath by way of the extraction passage, through the vane set, and then back into the flowpath by way of the injection passage. The vane set redirects the fluid, exchanging its tangential momentum for increased axial momentum so that the injected fluid is more favorably oriented than the extracted fluid.
Despite the merits of the vaned passage casing treatment, it is not without certain drawbacks. The vaned passageway consumes an appreciable amount of space, a clear disadvantage considering the space constraints typical of aerospace applications. The treatment also presents manufacturing and fabrication challenges. Moreover, debris may clog portions of the vaned passageway, compromising the effectiveness of the treatment. Finally, the treatment degrades compressor efficiency by allowing pressurized fluid to recirculate to a region of lower pressure in the compressor flowpath. The efficiency loss may be mitigated by employing a regulated system as proposed in U.S. Pat. No. 5,431,533. However the regulated system introduces additional complexity.
Finally, U.S. Pat. No. 5,586,859, also assigned to the assignee of the present application, discloses a "flow aligned" casing treatment in which a circumferentially extending plenum communicates with the flowpath by way of discrete extraction and injection passages. The flow aligned treatment, like VPCT, recirculates pressurized fluid to a lower pressure region, introducing the fluid into the flowpath in a prescribed direction to achieve optimum performance. However the flow aligned casing treatment suffers from many of the same disadvantages as VPCT.
Notwithstanding the existence of the above described casing treatments, compressor designers continually seek improved ways to reliably enhance compressor stability and minimize any attendant efficiency loss without complicating manufacture of the compressor or its components.